The connection between adjacent Land regions separated by water has always being overcome by building either suspended or laid bridges, which have secured the continuity of transport either by railways or motorways.
However, when the width of water stretch reaches high values or when, owing to the nature of water body floors or environmental conditions, the construction of bridges is not technically feasible. The transport of both goods and people is performed by naval or air means, with obviously higher costs and drawbacks essentially due to the long times required for boarding and landing.
Now, the need for rendering faster and cheaper, transport together with technological development, has led to the development of new connection systems, represented by underground or underwater tunnels. Typical examples are the underground tunnel excavated under The Channel, or the underwater tunnel for the metropolitan railway, submerged and laid on the sea bed of San Francisco Bay in California.
The underwater tunnels, generally constituted by a plurality of modules assembled with one another, can be laid on a water bodies' floor and anchored to it, or they can be floating inside water and anchored to the sea bed by means of tensioned elements in order to counteract their buoyancy. In both cases, the tunnels are subject to external forces which are constant in time (for example the forces due to the action of marine streams), or forces of periodical or random character (such as those which are due to heat contraction/expansion caused by temperature changes), or those due to the action of a seismic event.
Whilst for underwater tunnels laid on a water body's bed the stresses due to the external forces do not constitute a problem, because the action of such forces generally is compensated for by the friction forces due to the supporting bed. In the case of floating underwater tunnels, suitable devices interposed between their ends and the land are necessary, which ensure that the whole structure will withstand the stresses caused by the above mentioned forces and make possible the absorption of displacement, in particular the axial displacements, which generally are larger than those met in the case of laid tunnels. All the above is essential in order to prevent the structure from undergoing undesired displacements.
Furthermore, it is necessary that during a seismic event, the connection joints with the land can move freely in order to avoid the ends of the tunnel being affected by axial forces, which would otherwise are impossible to withstand. Under such conditions, the axial constraint between each tunnel end and the land must be equivalent to a spring and damper installed in parallel ("damped spring").